It is prior art technique to produce glass knives from a plate of flat glass which is placed with one plane surface on first supporting means providing at least two separate points of support symmetrically arranged in relation to the desired dividing line. The glass plate is placed in a predetermined dividing position by positioning means cooperating with the side edge surfaces of the plate. The positioned glass plate is fixed by fixing means being pressed against the plate which is then provided with the scratch forming the dividing line. Two further supporting means, which also are symmetrically arranged in relation to the desired dividing line and which have a line of symmetry perpendicular to the line of symmetry of the first supporting means, are applied against the second plane surface of the glass plate. Preferably, the second supporting means also constitute the said fixing means. One of the above-mentioned lines of symmetry is located in the desired dividing plane on the unscratched face of the glass plate. In a preferred embodiment, the last-mentioned line of symmetry is established by the first supporting means which usually comprise two separate, rounded supporting pins whose center points are located on said line of symmetry but which may also consist of an elongate edge or the like, as readily understood. Finally, the glass plate is broken by moving the first and second supporting means towards one another in a direction perpendicular to the plane of the glass plate, usually by moving the two second supporting means downwards towards the first supporting means until the glass plate is broken along the dividing line.
When producing glass knives, it is customary to start with a straight glass strip with plane-parallel longitudinal side edges and end side edges, which is divided into smaller square or rhombic glass plates of predetermined size. Positioning is achieved by causing one end edge surface of the glass strip to engage with an abutment pin, and by causing one longitudinal edge surface of the strip to engage with an abutment rail. The abutment pin and the abutment rail are situated each on one side of the dividing line. The position of the plate is maintained during the scratching and breaking operations.
The resulting smaller glass plates are then broken, substantially along a diagonal line, to produce triangular glass knives, the glass plate being positioned by causing the side edge surfaces at each of the two opposite corners between which the dividing line is to run, to engage with adjustable, fork-like means for positioning said corners. The engagement between these means and the glass plate is maintained during the scratching and breaking of the plate.
Swedish Patent SE 310,768 and U.S. Pat. Nos. 3,207,398 and 3,494,521 disclose examples of the above prior art technique. Apparatuses using this technique have long been manufactured and sold by LKB-Produkter AB, Bromma, Sweden under the name of KNIFEMAKER.
Although this known technique has functioned well and resulted in high-quality glass knives, a considerable number of the thus obtained plates and knives have not been up to standard and have had to be scrapped.